[Endothelial cells in the blood vessels of mature human placental villi].

In the microcirculatory system of mature human placental villi (small arteries, arterioles and venules respectively, pre- and postcapillaries, capillaries, and postcapillary venules) two endothelial cell types do exist. Numerous micropinocytotic vesicles are common to both types. In addition type I cells are distinguished by bundles of filaments which are probably contractile. Type II cells, however, are marked by a highly developed rough endoplasmatic reticulum, Golgi apparatus, and secretory vesicles. In all vascular parts examined type I cells predominate. The number of endothelial cells with secretory granules decreases in the direction of the venous vascular bed.